1. Field of the Invention
The invention relates to an intake flow rate detecting apparatus of an internal combustion engine which detects a rate of intake air flowing through an intake passage while compensating for intake pulsation and the like in the intake passage of the internal combustion engine for a vehicle, and a method thereof.
2. Description of the Related Art
In order to monitor an operation state of an internal combustion engine for a vehicle and perform various controls of the engine, a flow rate sensor is frequently provided upstream of a throttle valve, and an intake flow rate of the engine is calculated based on an output from this flow rate sensor.
As such a flow rate sensor, a vane air flow meter was previously used. This vane air flow meter includes a vane which is pushed to open due to a pressure difference that is generated when intake air passes therethrough, and the intake flow rate is measured by detecting the vane opening using a potentiometer or the like. However, this vane air flow meter has some drawbacks. For example, responsiveness is poor since the intake flow rate is measured based on the vane opening, and pressure loss in the intake passage is increased by the vane. Therefore, the most commonly used type of flow rate sensor at present is a thermal air flow meter in which a heater whose temperature is lowered by air flowing through the intake passage is embedded in a bridge circuit, and which measures the intake flow rate by compensating for the temperature of the heater. Unlike the vane air flow meter, since this thermal air flow meter does not include any moving parts, it does not have the above-mentioned drawbacks. In addition, since intake air weight is measured by this thermal air flow meter, calculation of the intake flow rate is less subject to a change in air density due to a change in the temperature or atmospheric pressure.
However, in the case of the above-mentioned thermal air flow meter, not only the rate of the air flowing toward a combustion chamber of an engine but also the rate of the air which is generated due to intake pulsation, that is, the rate of the air flowing in the reverse direction, is detected as the rate of the air which is taken into the combustion chamber. Therefore, when intake pulsation is generated, such a thermal air flow meter is directly affected by the intake pulsation, and accordingly, an output therefrom tends to be larger than the actual intake flow rate.
However, in a low load state in which the opening of the throttle valve is small, even when the intake pulsation is generated, this throttle valve itself functions as a wall which prevents the air flowing in the reverse direction from reaching the air flow meter. Accordingly, even in the case of the thermal air flow meter, the detection error is relatively small. However, in a high load state in which the opening of the throttle valve is large, the effect from the intake pulsation cannot be ignored. Namely, in this case, when the intake pulsation is generated due to opening or closing of the intake valve, the air flowing in the reverse direction due to the intake pulsation easily reaches the air flow meter through the throttle valve that is open wide. Consequently, an error in a detection performed by the thermal air flow meter becomes large.
Therefore, conventionally, in order to compensate for such an error in detection performed by the thermal air flow meter (hereinafter, referred to as “thermal air flow meter detection error”), a measure such as obtaining a pulsation correction coefficient whose parameters are a throttle opening and an engine speed, and multiplying the output from the air flow meter by the correction coefficient are being taken.
Recently, various mechanisms which control an air flow rate in the intake passage, particularly an air flow rate between the combustion chamber and the throttle valve, are frequently mounted on the internal combustion engine for a vehicle. Examples of such a mechanism, that is, a flow rate control mechanism, are a swirl control valve which generates a swirl flow in the air flowing into a cylinder (a combustion chamber), a variable valve mechanism which changes valve characteristics such as opening or closing time of an engine valve (an intake and exhaust valve), a lift amount thereof and the like, and a variable intake mechanism which changes an effective length of the intake passage. When such a flow rate control mechanism is provided, the intake pulsation often changes in an unexpected manner. Accordingly, even when the pulsation correction coefficient is obtained based on only the throttle opening and engine speed, as mentioned above, an error in detection performed by the air flow meter (hereinafter, referred to as an “air flow meter detection error”) may not be able to be appropriately compensated for.
Not only the thermal air flow meter, but also an air flow meter which equally detects air flow due to intake pulsation in both directions and which outputs detection signals according to these flow rates, has the above-mentioned problem in which a degree of accuracy of compensation for the detection error is decreased due to the mounting of the flow rate control mechanism.